In order to enhance yield, managed forestry encourages trees of a usable size to develop more rapidly by thinning out dense growths. It is desirable to shred the brush and trees removed in this process in situ, rather than haul it out for disposal. In this way, the residue creates mulch that protects and eventually enriches the soil. For the same reasons, it is also desirable to shred logging slash. In addition, logging roads and fire lanes must be cleared and maintained. In short, there are many applications in forestry for a machine with the capability to fell and/or shred trees. Highway right-of-ways, railroad right-of-ways and pipeline and power transmission line easements must be also cleared initially and periodically thereafter.
Such broad demand has generated a variety of land clearing machines, most frequently in the form of attachments to dedicated track mounted or rubber tired carriers. These machines are made in two basic configurations, either vertical or horizontal shaft. The former has its main shaft in a vertical position, like an over-sized rotary lawn-mower, the latter has its main shaft in a horizontal position, parallel to the ground like a reel-type lawn mower. In either case, the cutting and shredding is done by blades rotating at high speed. These blades may be fixed or swinging in relation to the main shaft. The fixed blade machines tend to break material up into larger chunks than the swinging blades, but the chunks are still small enough to divide the fuel for a potential fire hazard for safety purposes. This satisfies the minimum Forest Service requirements for reducing the rate of spread of a fire and making it easier to bring under control. However, most of the available equipment uses swinging blades, which generally rotate at higher speeds than would be the rule with fixed blades, and produce finer chips. The swinging blade machines require more horsepower but have a higher production rate and can be significantly lighter than an equivalent fixed blade unit. Swinging blades are also much more tolerant of impact against rocks, in that less damage is done to the blade and less shock loading is reflected back into the drive line. These factors weigh heavily in favor of swinging blade machines for all mobile, prime mover mounted applications.
As a general rule, the horizontal shaft machines are preferred in most applications. Since the horizontal shaft machines throw chips and debris only in a forward direction, they are considered to be safer than the vertical shaft machines, which tend to throw debris in all directions. These machines are also capable of cutting closer to the ground, can be more closely coupled to the carrier, and provide better operator visibility. However, lighter weight and closer coupling are relative properties, and the overhung load imposed by one of these units mounted at the front end of a carrier is such that either an oversized carrier or counterweighting may be required. Overall attachment weight is thus a significant consideration in the design of these machine attachments, along with capital cost, operating cost, performance, efficiency and durability.
These machines are expected to fell unwanted trees, cut fairly close to the ground, and shred whatever stands in the way. Such work is a continuum of abusive shock loads, a condition which must be carefully addressed in every structural and dynamic aspect of machine design. If weight and cost are of no concern, durability can be achieved by oversizing all of the components. Durability can also be achieved by placing load limiting slipclutches or shear pins at every critical point in the machine, but only if performance is of no concern. In short, every design decision is a compromise between conflicting requirements. That being said, the ideal machine will be inexpensive to manufacture, have no excessive structure or power, and perform adequately on the available power, with excellent efficiency. The challenge is to approach the ideal as closely as possible. To minimize excess structure it is obviously necessary to avoid over-sizing the component parts or using inappropriate materials. More than that however, the configuration of the machine, and each of its component parts, must be critically evaluated for structural and functional efficiency. If a part can be made to serve dual purposes, it may be possible to eliminate, or drastically reduce, the weight of a second part.
As an example, in some prior art, horizontal shaft machines it is recognized that the protective cover that guards the rotor and confines debris dispersal also serves as a structural frame component. By incorporating bent flanges and closed sections into the shroud shape, its stiffness and structural contribution is greatly enhanced while, at the same time, manufacturing costs are reduced. The present invention extends the scope of such opportunities.
A first object of the present invention is therefore, to provide a machine attachment capable of felling and shredding trees up to twelve inches in diameter in one pass. A second object is to provide this machine in a lightweight, readily manufactured configuration that will be relatively inexpensive to acquire and maintain. A third object is to provide such a machine in a durable form, that can run at least 2,500 hours between scheduled overhauls.